The normally crystalline vinylidene chloride polymers, i.e., those polymers containing at least about 70 weight percent of vinylidene chloride in the polymer molecule, and which exhibit crystallinity when examined by X-ray diffraction methods or between crossed Nicol prisms and compositions prepared therefrom by the inclusion of modifying agents, which composition retain the working characteristics of the crystalline polymers, are known to possess desirable physical and chemical properties such as resistance to ignition and combustion, toughness and insolubility in common solvents. Such polymers are generally useful in the formation of synthetic fibers, yarns, coatings and films.
Heretofore, however, such polymers have not been adaptable to conventional extrusion techniques used for the preparation of foamed structures due, primarily, to the inherent insolubility in these polymers of compounds conventionally used as blowing agents and to their high softening point, i.e., softening points above 100.degree.C, coupled with the propensity to degrade with the evolution of HCl, at temperatures only slightly above those temperatures necessary for polymer fabrication.
Prior attempts to prepare foamed normally crystalline vinylidene chloride polymers include, the work of T. W. Sarge and F. H. Justin (U.S. Pat. No. 2,695,427, issued Nov. 30, 1954) which produced a vinylidene chloride polymer foam of open-cell structure having a density as low as about 15 pcf by forming an admixture of a normally crystalline vinylidene chloride polymer and from about 1 to 15 percent by polymer weight of an alkali metal bicarbonate, then subjecting such mixture to a radio frequency field until the mixtures attained a temperature less than 200.degree.C but at least as high as the softening point of the polymer, and removing the resulting foamed product from the field before the copolymer degraded significantly. Such process, however, never attained commercial significance.
Related technology is also set forth in U.S. Pat. No. 2,948,048, issued Aug. 9, 1960. This patent contemplates a process for the preparation of oriented filaments composed of a normally crystalline vinylidene chloride polymer, such filaments having elongated microscopic voids throughout to provide a satiny appearance. These materials were obtained by blending trisodium aconitate with the polymer then extruding such blend as a monofilament. The monofilament was subsequently super-cooled, then warmed slightly and stretched to impart orientation thereto. Further, U.S. Pat. No. 3,322,695, issued May 30, 1967 contemplates the preparation of rigid cross-linked polymeric macroporous beads (including vinylidene chloride polymer beads having a plurality of interconnecting pores) by polymerization of the necessary monomer in the presence of from about 1/2 to about 20 times the weight of monomers of a solvent which is miscible with the unpolymerized material and exhibits limited solubility for the polymeric form of the monomeric material. Still further, U.S. Pat. No. 3,751,377, issued Aug. 7, 1973 contemplates processing vinylidene chloride copolymers wherein a volatile fluid foaming agent, which is generally a non-solvent for the resin, is admixed with the heat plastified resin and passed through an interfacial surface generator prior to being extruded into foam form.
It is the primary objective of the present invention to provide an industrially suitable means for preparing extruded foam from normally crystalline vinylidene chloride polymer wherein such foam is highly stable, i.e. shrinkage resistant; of relatively low density, i.e., having a density of less than about 15 pcf; is desirably resistant to chemicals, ignition and combustion; has low water vapor and oxygen transmission rates, and has high heat distortion values, i.e., can resist temperatures up to about 150.degree.C.